Fix CRF decompile failing on c0.30 (#1384)

- ClassLineNumbers.readMappings throws a nicer error when the input data is empty.
- GenerateSourcesTask include linemap filename when failed to read.
- CRF decompiler wont write empty linemap data.
- Add test

src/decompilers/cfr/net/fabricmc/loom/decompilers/cfr/LoomCFRDecompiler.java

@@ -98,6 +98,10 @@ public final class LoomCFRDecompiler implements LoomInternalDecompiler {
 	}
 
 	private void writeLineMap(Path output, Map<String, Map<Integer, Integer>> lineMap) {
+		if (lineMap.isEmpty()) {
+			return;
+		}
+
 		try (Writer writer = Files.newBufferedWriter(output, StandardCharsets.UTF_8)) {
 			for (Map.Entry<String, Map<Integer, Integer>> classEntry : lineMap.entrySet()) {
 				final String name = classEntry.getKey().replace(".", "/");

src/main/java/net/fabricmc/loom/decompilers/ClassLineNumbers.java

@@ -28,10 +28,7 @@ import static java.text.MessageFormat.format;
 
 import java.io.BufferedReader;
 import java.io.IOException;
-import java.io.UncheckedIOException;
 import java.io.Writer;
-import java.nio.file.Files;
-import java.nio.file.Path;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
@@ -53,14 +50,6 @@ public record ClassLineNumbers(Map<String, ClassLineNumbers.Entry> lineMap) {
 		}
 	}
 
-	public static ClassLineNumbers readMappings(Path lineMappingsPath) {
-		try (BufferedReader reader = Files.newBufferedReader(lineMappingsPath)) {
-			return readMappings(reader);
-		} catch (IOException e) {
-			throw new UncheckedIOException("Exception reading LineMappings file.", e);
-		}
-	}
-
 	public static ClassLineNumbers readMappings(BufferedReader reader) {
 		var lineMap = new HashMap<String, ClassLineNumbers.Entry>();
 
@@ -81,6 +70,7 @@ public record ClassLineNumbers(Map<String, ClassLineNumbers.Entry> lineMap) {
 
 		CurrentClass currentClass = null;
 		Map<Integer, Integer> currentMappings = new HashMap<>();
+		boolean didRead = false;
 
 		try {
 			while ((line = reader.readLine()) != null) {
@@ -88,6 +78,8 @@ public record ClassLineNumbers(Map<String, ClassLineNumbers.Entry> lineMap) {
 					continue;
 				}
 
+				didRead = true;
+
 				final String[] segments = line.trim().split("\t");
 
 				if (line.charAt(0) != '\t') {
@@ -108,7 +100,10 @@ public record ClassLineNumbers(Map<String, ClassLineNumbers.Entry> lineMap) {
 			throw new RuntimeException(format("Exception reading mapping line @{0}: {1}", lineNumber, line), e);
 		}
 
-		assert currentClass != null;
+		if (!didRead) {
+			throw new IllegalArgumentException("Unable to read empty linemap data");
+		}
+
 		currentClass.putEntry(lineMap, currentMappings);
 
 		return new ClassLineNumbers(Collections.unmodifiableMap(lineMap));

src/main/java/net/fabricmc/loom/task/GenerateSourcesTask.java

@@ -631,6 +631,8 @@ public abstract class GenerateSourcesTask extends AbstractLoomTask {
 
 		try (BufferedReader reader = Files.newBufferedReader(linemapFile, StandardCharsets.UTF_8)) {
 			return ClassLineNumbers.readMappings(reader);
+		} catch (Exception e) {
+			throw new IOException("Failed to read line number map: " + linemapFile, e);
 		}
 	}
 

src/test/groovy/net/fabricmc/loom/test/integration/DecompileTest.groovy

@@ -106,4 +106,30 @@ class DecompileTest extends Specification implements GradleProjectTestTrait {
 		result2.task(":genSourcesWithVineflower").outcome == SUCCESS
 		result3.task(":genSourcesWithVineflower").outcome == SUCCESS
 	}
+
+	// https://github.com/FabricMC/fabric-loom/issues/1362
+	@Unroll
+	def "CFR legacy"() {
+		setup:
+		def gradle = gradleProject(project: "minimalBase", version: PRE_RELEASE_GRADLE)
+		gradle.buildGradle << '''
+				loom {
+					noIntermediateMappings()
+					clientOnlyMinecraftJar()
+				}
+
+                dependencies {
+                    minecraft "com.mojang:minecraft:c0.30_01c"
+					mappings loom.layered() {
+						// No names
+					}
+                }
+		'''
+
+		when:
+		def result = gradle.run(task: "genSourcesWithCfr")
+
+		then:
+		result.task(":genSourcesWithCfr").outcome == SUCCESS
+	}
 }